This invention relates to an improved armature assembly in a D'Arsonval type instrument.
It is well known that electrical instruments have very broad use in the electrical measuring art. Because of the large number of measurements which are made in regulating processes and apparatus operated electrically there is need for large numbers of instruments. Because of the large number there is a premium on the smaller size instruments so that the instruments can be grouped to give readily available information to persons using the instruments.
Further, because of the very large number of such instruments which are used and the many purposes and applications in which they are employed, it is desirable that they be made at low cost and yet with high reliability and accuracy in performance.
Instruments of the D'Arsonval type include a moving armature or coil assembly which is mounted for rotation through the field of a permanent magnet assembly. When the armature assembly is energized by means of a current flowing therethrough, the resulting magnetic fields interact to produce a torque which rotates the armature assembly relative to the permanent magnet assembly. An instrument pointer is generally connected to the movable armature assembly to yield a readout with respect to a faceplate or scale mounted behind the pointer as the torque is a function of the current magnitude.
One of the requirements for accuracy in electrical instruments of the D'Arsonval type as provided pursuant to the present invention is that they have small profile and yet be capable of producing reproducible readings from one meter to another. In part this is dependent on having a suspension mechanism for the armature assembly which is of desirably low profile, is sturdy and yet which can be produced with great reliability at low cost and in the desirable small size.
Of particular importance is the structuring of the armature assembly so that it will pivot with reliable low resistance to turning. In this respect the pivot mechanism, that is the means on which the armature assembly turns, must have both a low resistance to turning and also a high reproducibility in such low resistance from one meter to another. Further, it is highly desirable that this reproducibility be maintained over an extended period of time.
Although the mechanism as described and claimed in the applications referenced above has proven to be highly satisfactory and has been manufactured and sold with a great degree of success in providing reliable economical meter units to the purchasing public, there are, of course, improvements which can be made in the structure and one of these improvements relates to the use of the structure in a more rigorous or demanding environment. One such environment is an environment in which the structure is vibrated, dropped or subjected to rapid changes in motion. In such cases it has been found that although the structure is reliable and has a long useful life as described above in normal circumstances, over a period of time, jarring, vibration, dropping or rapid motion can change the reading from the device as it can cause some distortion of the bobbin about which the sensing coil is wound to form an armature. The bobbin is itself preferably an article of relatively lightweight construction and may be made of aluminum in a preferred embodiment. The bobbin has an elongated shape to achieve the desired low profile. The aluminum bobbin also serves as a single winding damping coil to obtain prompt response from the instrument in measuring. If a less flexible aluminum is used, there is a loss of conductivity and a corresponding loss of damping. In spite of the light weight of the aluminum bobbin and sensing coil, when the overall structure is subjected to the rapidly changing motion as described above, there is some tendency for the bobbin to fold inward so that the points of the pivot needles do not seat as precisely in their jeweled pivot sockets as is desirable. Accordingly, there can be a loss of accuracy if the article is subjected to very rapid motion changes such as occur in strong vibration, dropping or other impacting.
It is, therefore, an object of the present invention to provide an accurate, shock-resistant D'Arsonval movement.
It is a further object of the present invention to provide an accurate, shock-resistant D'Arsonval movement having a low profile.